A silicon photomultiplier (SiPM: Silicon Photomultiplier) is a photoelectric conversion element electrically operatable in the same manner as a photomultiplier tube (PMT) or a micro-channel plate (MCP).
As illustrated in FIG. 16, an SiPM is formed by a two-dimensional assembly of microcells composed of photodiodes (PD).
The microcell is composed of PDs with bias voltage of breakdown voltage or more applied thereto and quenching resistors for recovery of the PDs with Geiger electric discharge phenomenon.
FIG. 17 illustrates a currently-used SiPM. As illustrated in FIG. 17, the PDs with same polarity are arranged in parallel. When light is incident on the SiPM, photons enter into some of the PDs in the macrocell where current is passed in the direction opposite to the polarity. Accordingly, all the current passing through the PDs travels toward an output terminal Iout. The output terminal Iout outputs current with intensity depending on a number of PDs into which the photons enter. See, for example, Japanese Patent Publication (Translation of PCT Application) 2010-536186A.